1. Field of the Invention
This invention generally relates to a thread tension regulating apparatus for sewing machines. More particularly, the invention relates to an apparatus for allowing an adjustment dial to be returned to its preset home position.
2. Prior Arts
When sewing machines using plural kinds of sewing threads, such as overlocking machines, are used to form various stitches (overlock stitching, loop stitching, fancy stitching, etc.), it is necessary to control or adjust tension to be applied to each sewing thread depending upon the kind of stitches to be formed.
A thread tension regulating or balancing apparatus capable of controlling tension to be applied to sewing threads is disclosed, for example by the Japanese Patent Public Disclosure No. 02-255176.
FIG. 6 shows an overlock sewing machine of the single needle type in which one needle thread and two looper threads are used. A body 1 of the sewing machine includes, in its front surface, a thread tension regulating units 2A, 2B and 2C for controlling tension in a needle thread A, an upper looper thread B and a lower looper thread C, respectively. A thread tension in each thread tension regulating unit may be set independently at desired values by actuating a regulation dial 3.
As shown in FIG. 7, the thread tension regulating units are of the same mechanical construction. Accordingly, only a single thread tension regulating unit will be explained below.
When the tension regulation dial 3 is rotated in one direction, a gear portion 3' formed integrally with the regulation dial 3 will rotate a thread tension regulator (referred to as a thread tension regulating gear 5 hereinbelow) through an intermediate gear 4. The thread tension regulating gear 5 is threadingly engaged with a thread tension shaft 8 fixedly carried by a conversion shaft 9, so that the gear is displaced axially, whereby clamping pressure exerted by tension discs 7a and 7b is changed due to compression or elongation of a thread tension spring 6.
A thread take-up spring thread guard 10 moves in its axial direction by the rotation of a conversion cam 17 corresponding to its cam surface, wherein its movement in the axial direction is confined by a stop ring 11 which is fitted on the thread tension shaft 8. The conversion cam 17 is fixed on the conversion shaft 9, and supports the tension disc 7b which is located at the opposite side of the thread tension spring 6.
Fixed to a base 12, are the thread tension shaft 8, fixed shafts 13 and 14, and a guide pin 15. The fixed shafts 13 and 14 are to support the regulation dial 3 and the intermediate gear 4, whereas the guide pin 15 is to prevent tension discs 7a and 7b as well as the thread take-up spring thread guard 10 from turning.
Reference numeral 16 designates a spring, and numeral 18 designates a conversion dial which is fixed on the conversion shaft 9. The spring 16 indicates the degree of thread tension when adjusting each thread tension unit to an adequate tension by the adjusting dial 3. The tension can be shown by aligning the standard scale (e.g. 5) of the scale 3b (with numerals 1 through 9) with the indicator 3a; The scale 3b is printed on the outer periphery of the regulation dial 3.
In this thread tension regulating apparatus, projections 17' are provided on part of the conversion cam 17 which rotates together with the conversion shaft 9. The conversion shaft 9 is moved by the conversion dial 18 to adjust the tension of plural thread tension units. The thread take-up spring thread guard 10 moves in the axial direction, causing the tension disc 7b which is on the opposite side of the thread tension spring 6 to move, and the clamping pressure of the thread tension discs 7a and 7b to change.
As explained above, each of the thread tension units can be manually adjusted by the regulation dial 3 when they are changed simultaneously.
Thread tension adjusting apparatuses of the aforementioned conventional type have the following problems.
When adjusting thread tension by the regulation dial to cope with changes of the sewing threads or sewing materials (hereinafter called the changes of the sewing mode), it is necessary to return the thread tension device to its original condition, i.e. the condition that provides a predetermined tension, for the next sewing after a sewing is finished. Returning the regulation dial, on which delicate adjustments have been made, to the original condition requires a complicated process that needs time and skills, causing a decreased efficiency of sewing work.
In addition, to see if the thread tension device has returned to its original condition, test sewing is needed, thus consuming unnecessary threads and sewing materials.